1. Field of the Invention
The present invention relates to a structure of a die plate for a granulating machine of resin pellets.
2. Description of the Related Art
A resin pellet-granulating machine includes a resin cutting knife (rotary blade) and a die plate, wherein nozzle holes are formed in a hardened surface provided on the die plate. In this structure, minute pellets are generated by rotating and sliding the resin-cutting knife on the hardened surface, while discharging a resin from the nozzle holes, to cut the resin.
(1) With respect to a die plate for such a granulating machine, Japanese Patent Laid-Open No. 5-124035 Publication discloses, as related arts, a structure in which recessed parts communicating with a resin passing nozzle are formed on the surface of a die base metal, a die surface hardened part (cemented carbide tip) by a preliminarily formed annular cemented carbide is provided in each recessed part, and the annular cemented carbide is adhered and fixed to the die base metal by brazing (FIG. 9 and Paragraph No. 0003 of Japanese Patent Laid-Open No. 5-124035 Publication).
(2) Japanese Patent Laid-Open No. 5-124035 Publication also discloses, as related arts, a structure in which a die surface hardened part is formed by thermally spraying a tungsten carbide-based self-soluble alloy with wear resistance in a predetermined thickness on to the surface of a die base metal having a plurality of resin passing nozzles (FIG. 8, Paragraph No. 0002 of Japanese Patent Laid-Open No. 5-124035 Publication).
(3) Japanese Patent Laid-Open No. 5-124035 Publication further discloses, as related arts, a structure in which a die surface hardened part composed of a titanium carbide (TiC) sintered plate of a predetermined thickness is formed on the surface of a die base metal (FIG. 10 and Paragraph No. 0004 of Japanese Patent Laid-Open No. 5-124035 Publication).
(4) A manufacturing method proposed by Japanese Patent Laid-Open No. 5-124035 Publication for the above-mentioned (1)-(3) comprises the steps of filling a powdery alloy of a component for forming a hardened layer in the recessed parts formed on the surface of the die base metal, and press-sintering the powdery alloy by means of hot isostatic pressurization (HIP) to form the die surface hardened part. Japanese Patent Laid-Open No. 5-124035 Publication describes that according to this, the die plate hardened part is integrated with the base metal by diffusion-bonding to exhibit a strong adhesive force. The strong adhesive force can prevent cracking or peeling of the die plate hardened part even if an operation extends over a long period.
[1] In the structure as shown in FIG. 9 of Japanese Patent Laid-Open No. 5-124035 Publication, however, a lot of base metal exposed parts are formed on the surface of the die in positions between tips. Accordingly, the base metal exposed parts are selectively worn by sliding the resin-cutting knife on the die surface. The wearing of the base metal is apt to cause dropping-off of the tips. Further, since one cemented carbide tip has one nozzle hole (so-called single-hole type tip), and such tips are arranged at intervals, the freedom in arrangement of nozzle holes on the die surface is low, and it is difficult to enhance the production efficiency by increasing the number of nozzle holes. In the single-hole type tip, brazing of a number of tips is required to form a number of nozzle holes. Accordingly, a tip-to-base metal or tip-to-tip (so-called boundary part) brazing exposed part is increased. Since a brazing filler metal is gradually lost by cavitation and corrosion, the dropping-off of the tips is easily caused by the loss of the brazing filler metal in a structure having many brazing parts.
[2] When the die surface hardened part is formed by thermally spraying the self-soluble alloy as in FIG. 8 of Japanese Patent Laid-Open No. 5-124035 Publication, the die surface hardened part can be formed only in a thickness of about 0.5-2 mm. The small thickness leads to shortening of the working life. Japanese Patent Laid-Open No. 5-124035 Publication also points out the effect that the life expires about 1 year. It is also difficult to obtain a uniform sprayed coating.
[3] In the structure in which the die surface hardened part is formed by the sintered plate of titanium carbide (TiC) as in FIG. 10 of Japanese Patent Laid-Open No. 5-124035 Publication, since the fixation of TiC of a wide area is technically difficult, a method for enhancing the hardness of a hardened surface, for example, by precipitation hardening treatment must be adapted. The precipitation hardening treatment is undergone after TiC is fixed and integrated to the die plate base metal by brazing, and nozzle holes are bored in a part which forms the hardened surface. Accordingly, a large number of processes are required.
On the other hand, although TiC alloy single-hole tips about 10 by 10 mm square may be brazed as many as nozzle holes, this method has a large number of processes similarly to the above method. And the cost necessary for working the single-hole tips is also increased because one-by-one brazing of the tips. In the structure in which one nozzle is provided in one tip and such tips are arranged at intervals, the freedom in the arrangement of nozzle holes on the die surface is low similarly to the above-mentioned (1).
[4] The method proposed by Japanese Patent Laid-Open No. 5-124035 Publication requires the formation of recessed parts for filling the powdery alloy on the surface of the base metal, and the hardened part cannot be formed in a very large size. Accordingly, in order to form many nozzles, a number of relatively small recessed parts must be formed in the base metal. Consequently, many base metal exposed parts are formed in the boundaries between recessed parts as shown in FIGS. 3 and 6 of Japanese Patent Laid-Open No. 5-124035 Publication, and the base metal exposed parts are selectively worn by sliding the resin-cutting knife on the die surface to easily cause the dropping-off of the tips.